1. Field of the Invention
This invention relates to a method for stabilizing the characteristics of a microorganism containing a plasmid (plasmid-having microorganism).
2. Description of the Prior Art
Attempts have been made to produce useful microorganisms by transforming Escherichia strains by inserting a plasmid which incorporates useful genetic information.
However, in general, the plasmid may be eliminated during culturing, so that the microbial characteristics controlled by the plasmid become unstable in the microbial cell population. This phenomenon restricts the commercial application of the plasmid-containing microorganisms.
Such instability can be diminished if the plasmid-containing cells alone survive, while the plasmid-free cells, arising by the above-mentioned phenomenon, are selectively killed.
Attempts have been made to produce this selective killing of cells which do not contain the desired plasmid by using a plasmid which incorporates one or more genes which endow the cell with resistance to antibiotics. The cells are then cultured in a medium containing the antibiotic(s) corresponding to the resistance factors conferred by the plasmid. In such a medium, only the cells which contain the plasmid can survive, since they alone have the ability to resist the antibiotic(s). If the cells lose the plasmid containing the commercially useful genes, they also lose their antibiotic resistance and succumb to the action of the antibiotics which are present in the culture medium. This method, however, suffers from the drawback that relatively large amounts of antibiotic must be used, and therefore it is not well adapted to industrial exploitation.
Hence, a need has continued to exist for a method of assuring the stability of a culture of microorganisms containing an industrially useful plasmid which is effective, economical, and adapted to industrial practice.